There have been conventionally known nonaqueous electrolyte solution type lithium secondary batteries using a nonaqueous electrolyte solution, such as an organic solvent, as an electrolyte solution and pure Li for a negative electrode. The use of a nonaqueous electrolyte solution and pure Li aims at achieving battery power having high energy density and high electromotive force. However, such batteries are associated with problems in that dendrite (branch-like crystals) easily grow on the surface of the pure Li negative electrode due to the electrodeposition caused by the discharge reaction between the negative electrode and Li ions at the time of charging. The growth of the dendrite markedly degrades the battery function, as well as short-circuits the positive and negative electrodes by penetrating a separator (electrolyte solution layer) to ultimately result in poor charge-discharge cycle life.
In an attempt to solve such problems, it has been proposed to form a negative electrode from an Li alloy comprising an intermetallic compound of Al, Bi, Pb, Sn, In etc. and Li. According to this method, the growth of dendrite is suppressed by decreasing the speed of discharge reaction between the negative electrode and Li ions. The decreased speed of discharge reaction is achieved by increasing the electrode potential of pure Li by alloying, thereby lowering the activity of the negative electrode. However, this also poses problems in that the lowered activity of the negative electrode by alloying also lowers electromotive force and charge-discharge capacity, and in that the negative electrode develops cracks through expansion and contraction in volume for absorption and discharge of Li during charge-discharge cycles, since the alloying makes the electrode fragile, and the negative electrode ultimately becomes pulverulent, thereby shortening the service life of the battery.
There has also been proposed a negative electrode formed from a dilute solid solution alloy comprising Mg, Ag, Zn etc. added to Li, for achieving high electromotive force and high charge-discharge capacity. However, the negative electrode formed from such dilute solid solution alloy does not differ significantly from the surface of pure Li in terms of electrochemical properties, since the aforementioned alloy components are sprinkled in the metal crystal matrix of Li, and the electrode is susceptible to dendrite growth during charging, like pure Li negative electrode. This type of lithium secondary battery is less practical due to its short charge-discharge cycle life.
It is therefore an object of the present invention to provide an alloy for lithium secondary battery negative electrode exhibiting high charge-discharge capacity, high energy density and less degradation due to the repetitive charge-discharge cycles, and a lithium secondary battery having high electromotive force, high charge-discharge capacity, high battery power of high energy density, and superior charge-discharge cycle life.